snow

snow, precipitation formed by the sublimation of water vapor into solid crystals at temperatures below freezing. Sublimation resulting in the formation of snow takes place about a dust particle, as in the formation of raindrops. Snowflakes form symmetrical (hexagonal) crystals, sometimes matted together if they descend through air warmer than that of the cloud in which they originated. Apparently, no two snow crystals are alike; they differ from each other in size, lacy structure, and surface markings. Snowfall, reduced to its liquid equivalent, is usually included in statistics on rainfall; the factors determining snowfall are similar to those affecting rainfall. On an average, 10 in. (25 cm) of snow is equivalent to 1 in. (2.5 cm) of rain. In the United States the average snowfall is about 28 in. (71 cm) per winter; the record is 1,140 in. (2,896 cm) at Mt. Baker in Washington state during the snow season of 1998–99. Snow that piles up on slopes may suddenly slide downward in an avalanche. A glacier consists of ice that was formed from compacted snow. Snow serves as an insulating blanket, lessening to some extent the extremes of temperature fluctuation to which the soil is subjected, but it also brings about a rapid cooling of the overlying atmosphere, giving rise to polar air masses. Snow lessens loss of water by dormant plants. The sudden melting of snow is a primary cause of floods. Snow necessitates the building of snowsheds over rail lines and highways in certain mountain localities where a heavy fall is likely to impede travel; the use of snowplows to clear sidewalks, streets, and roads; the use of snow fences to prevent drifting over roads; and the use of skis, snowshoes, toboggans, snowmobiles, and sleds for travel. It is a primary factor in the location of winter sports centers and so has great economic value to certain areas. In some ski resorts machines are used to make artificial snow. As in the case of rainfall, snowfall has been produced artificially by introducing dry-ice pellets into supercooled clouds, that is, clouds containing unfrozen water droplets at temperatures below freezing.